1. Field of the Invention
The present invention relates to a method of controlling an RF pulse in a magnetic resonance imaging apparatus and a system for practicing the method.
2. Description of the Related Art
In a magnetic resonance (MR) phenomenon, an atomic nucleus having a magnetic moment in a static field absorbs or emits an electromagnetic wave having a specific frequency by resonance. A resonance frequency (Lamor frequency) .nu..sub.0 of the atomic nucleus is represented by the following equation: EQU .nu..sub.0 =.gamma.H.sub.0 /2 (1)
where .gamma. is the magnetogyric ratio unique to the type of atomic nucleus, and H.sub.0 is the intensity of a static field.
An MRI (magnetic resonance imaging) apparatus for diagnosing the inside of a subject to be examined by utilizing the MR phenomenon applies a slicing gradient field and an RF (radio frequency) pulse to the subject placed in a uniform static field. A nuclide of interest in the subject is selectively excited to generate an MR signal. In order to add position information to the MR signal, a phase encoding gradient field is applied in a direction perpendicular to the directions of the slicing and phase encoding gradient fields. The MR signal generated in the subject is received by an RF coil and is subjected to image processing, thereby displaying an MR image.
In a transmission system of an MRI apparatus shown in FIG. 1, an oscillator 51 generates a high-frequency signal having a frequency corresponding to the Lamor frequency of a nuclide of interest. A modulator 52 modulates the high-frequency signal output from the oscillator 51 by using a modulation signal based on a pulse sequence. An amplifier 53 amplifies the high-frequency signal modulated by the modulator 52 up to about 1 to 15 kW. The amplified high-frequency signal is applied as an RF pulse by an RF coil 54 to a subject to be examined.
In the transmission system having the above-described arrangement, the slice profile of an MR slice image is determined by the input/output characteristics of the amplifier 53. Input and output signals to/from the amplifier 53 may exhibit waveforms shown in, e.g., FIG. 2 depending on a variation in components of the amplifier 53. In FIG. 2, reference symbol IN denotes an input signal; and OUT, an output signal. That is, as shown in FIG. 3, although a linear input/output characteristic curve S.sub.L is preferably obtained, a nonlinear input/output characteristic curve S.sub.N is obtained in practice.
A general amplifier can perform substantially linear amplification with a predetermined amplification factor when an input signal has a low level. However, when an input signal has a high level, since the amplification factor is decreased, linear amplification cannot be performed. In a conventional MRI apparatus, however, proper countermeasures against a nonlinear amplification or occurrence of distortion of an output signal in a high-frequency amplifier have not been taken.
When an RF pulse is generated by supplying a distorted current to an RF coil, the RF pulse excites not only a nuclide of interest in a selective excitation portion in a subject to be examined but also nuclides in other portions. Since signals from other portions are thus included in a detected MR signal, an MR image having high resolution cannot be reconstructed.
FIG. 4A shows the spectrum distribution of an RF pulse obtained by ideal linear amplification using a high-frequency amplifier. In FIG. 4A, a spectrum width .DELTA..omega. corresponds to the thickness of a portion to be excited. If linear amplification is not performed in a high-frequency amplifier, the spectrum of an obtained RF pulse exhibits a spectrum distribution shown in, e.g., FIG. 4B. As described, therefore, in this case, not only a selected excitation portion of a subject to be examined but also an adjacent portion are excited. For this reason, in a multi-slice operation, gaps are generated between the respective slices.
A strong demand, therefore, has arisen for an MRI apparatus having a high-frequency amplifier capable of generating an RF pulse without distortion.